You want the 40% peroxide that they use in hair salons. Don't use bleach....it will eat the bone. Soak the skull in the peoxide for a couple of hours, rinse it off real good and lay it in the sun to dry. The heat from the sun will help to whiten it. Once it is dry, then spray it with the clear coat if you want. You may want to soak it in regular ammonia for about a week to remove any remaining grease or oils left on the skull prior to the peroxide soak. If you remove them, it help to keep it from yellowing later on.